A beekeeper's entire colony has died suddenly in the Sandy area. It's the fourth unexplained hive die-off in Clackamas County in less than a week.

And for the second time in a week – this time in Beaverton –bees were found dead under flowering linden trees.

Linden trees sprayed with a class of pesticides called neonicotinoids can harm bees if sprayed improperly.

"It feels like the beginning of summer is now marred by an onslaught of bee kills. It's very concerning," said Aimee Code, pesticide program coordinator for the Xerces Society, a Portland-based insect conservation group.

State agriculture officials, the Oregon State Beekeepers Association, the Xerces Society and scientists from Oregon State University all are involved in investigating the deaths.

"We are very interested in knowing what's happening so things can be properly addressed," said Paul Anderson, president of the beekeeper's association.

U.S. Rep. Earl Blumenauer (D-Portland), a strong advocate for pollinator health, is stepping up calls for a moratorium on certain pesticides until they can be better studied. So is the Eugene-based advocacy group Beyond Toxics.

"The main thing is there doesn't seem to be any safe use of neonicotinoids," Lisa Arkin, executive director of Beyond Toxics, said.

The string began June 17, when residents of a north Eugene apartment complex found sidewalks littered with dead and dying bees.

The Oregon Department of Agriculture found that Glass Tree Care and Spray Service sprayed neonicotinoid pesticides on flowering linden trees, in violation of current labeling laws.

The state suspended the company's license and required it to cover the trees with shade cloth to keep the bees away, ODA spokesman Bruce Pokarney said Tuesday.

It likely will impose a civil penalty as well, he said.

Over the next few days, three Clackamas County beekeepers began finding dead and dying hives. An ODA investigator took samples from those last week. Results are not back yet, Pokarney said.

On Friday, beekeeper Austin Bennington discovered a sudden hive failure. ODA was at his property today to investigate.

"I've kept bees at that location for six years without any problems," Bennington said. "It's disappointing, that's for sure."

Late Monday, hundreds of dead bees were discovered along a Beaverton road, under flowering linden trees. ODA took samples of those as well.

There is no evidence that the colony failures are related, Pokarney said.

Sudden colony death could be caused by pesticides, but it also could be due to other factors, Anderson, of the beekeeper's association said.

"Pollinator decline is a grave concern for farmers and consumers world-wide," Blumenauer said in a statement. "Incidents like the one in Eugene illustrate that it's important to take a closer look at pesticide use and why passing our Save America's Pollinators Act is so urgent. I was pleased to see the White House call for the establishment of a Pollinator Health Task Force, which is a positive step. I will continue to urge the EPA to both expedite its review of neonicotinoid pesticides, and, in the meantime, suspend their use until the agency has thoroughly demonstrated that these pesticides do not pose a threat to pollinators."